Linear voice coil actuator with compensating coils

ABSTRACT

An actuator is provided in which one or more separate compensating coils are employed to alter the distribution of the magnetic flux density in the air gap to produce a substantially constant force throughout the stroke; and in which current in the separate compensating coils can be adjusted with the stroke using separate power supplies and position feedback.

RELATED APPLICATIONS

[0001] The present application claims priority under 35 U.S.C. §119(e)from provisional application number 60/192,999 filed Mar. 28, 2000.

TECHNICAL FIELD

[0002] The present invention is directed generally to linear actuators,and more particularly to compensating magnetic flux generated by thecurrent in the moving coil of voice coil actuators.

BACKGROUND ART

[0003] Voice coil actuators are electromagnetic devices which provideforce proportional to current applied to a coil. U.S. Pat. No.5,677,963, assigned to the assignee of the present application, is aexample of such voice coil actuators, and incorporated by referenceherein in its entirety.

[0004] Different configurations of linear voice coil actuators canprovide different shapes of the Force vs. Stroke curves. However, theseshapes cannot be changed without increasing the power level in themoving coil. But even this measure achieves only limited results due toa given distribution of the primary magnetic field and the armaturereaction.

[0005] In a servo system, it may be necessary to maintain constant forceor constant speed within a certain stroke. U.S. Pat. No. 5,177,383,entitled “Voice coil motor,” discloses coils with shorted turns employedin an attempt to achieve such characteristics. Because the turns inthose coils are shorted, however, current in those turns cannot beadjusted when the moving coil moves from one position to another.

SUMMARY OF THE INVENTION

[0006] In accordance with the present invention, one or more separatecompensating coils are employed to alter the distribution of themagnetic flux density in the air gap to produce a constant forcethroughout as much stroke as possible. Current in the separatecompensating coils can be adjusted with the stroke using separate powersupplies and a position feedback.

[0007] These and other features of the present invention will be morereadily appreciated upon consideration of the following detaileddescription and accompanying drawings.

DESCRIPTION OF THE DRAWINGS

[0008]FIG. 1 is a simplified cross section of a closed ended linearvoice coil actuator embodiment of the present invention.

[0009]FIG. 2 is a simplified cross section of an open-ended linear voicecoil actuator embodiment of the present invention.

[0010]FIGS. 3 and 4 depict required magneto-motive force in Ampere-turnsvs. stroke for a given constant value of the output force ofcompensating coils positioned in at the ends of the magnets, and aroundthe core, respectively, in accordance with the present invention.

[0011]FIGS. 5 and 6 depict Force vs. Stroke characteristics with twodifferent types of compensating coils in accordance with the presentinvention.

[0012]FIG. 7 is a simplified functional block diagram illustrating theconnection of the compensating coils to separate power supplies with thevalue of the compensating current to the compensating coils beingcontrolled as a function of stroke in accordance with the presentinvention.

DETAILED DESCRIPTION

[0013] Referring to FIG. 1, in accordance with the present invention,compensation coils, for example 10-A, can be wound directly around thecore 12; or compensation coils, for example 10-B and 10-C, can belocated in the cavities, for example cavities 14 and 16, between themagnets 18 and inside surfaces 20 of the field blank 22 of the actuator.Compensating coils 10-A, 10-B and 10-C can be used in closed-endedactuators, as shown in FIG. 1, as well as open-ended actuators, as shownin FIG. 2.

[0014] As can be seen in the closed-ended embodiment of the inventionillustrated in FIG. 1, a core compensating coil 10-A is provided. Apermanent magnet 18 is positioned with respect to the core 12 so thatthe longitudinal axis of the permanent magnet 18 is substantiallyparallel with the axis of the core 12. A field blank 22 made from softmagnetic material is used to form a magnetic circuit between the ends ofcore 12 and around permanent magnet 18.

[0015] A voice coil actuator in accordance with the present inventioncan have a variety of shapes, including a rectangular configuration anda cylindrical configuration. A rectangular embodiment will be describedin the following sections, as an example, it being understood that othershapes are contemplated within the spirit of the present invention. Aswill be apparent to those skilled in the art, although not shown in thefigures, brackets or other linkages can be used to connect the movingcoil to the load of the actuator. These brackets or other linkages canextend through longitudinal slots formed in the field blank 22 of theactuator.

[0016] In the embodiment of FIG. 11 permanent magnet 18 has arectangular shape. The field blank 22 has a rectangular shape and closedends. Permanent magnet 18 is positioned on the inside surface of thefield blank and has a length which is selected so that cavities 14 and16 are defined between the inside surfaces 20 of the ends of the fieldblank 22 and the permanent magnet 18. The inner dimension of permanentmagnet 18 and the outer dimension of core 12 are selected so that a gapis formed between them. The moving coil 24 is constructed to bepositionable in and to be moveable along this gap. Although not shown,those of ordinary skill will recognize that the moving coil 24 can becontrolled through connections which extend to the exterior of fieldblank 22, in a conventional manner.

[0017] In the closed-ended embodiment of the present invention,compensating coils 10-B or 10-C can be positioned in one or both of thecavities 14 or 16, respectively, which have been formed between thepermanent magnet 18 and the inside surfaces 20 of the field blank 22.Alternatively, a core compensating coil 10-A can be employed which iswound around the core. The core compensating coil 10-A can beconstructed to extend substantially the entire length of the core 12.

[0018] The structure provided in FIG. 1 permits the altering of thedistribution of the flux density in the gap. In turn, this permits thecontrol of the flux density as a function of the stoke. Thus, bycontrolling the magneto-motive force (MMF) in the compensating coils,one is able to control the characteristics of the actuator to have, forexample constant force across the stroke, or other desired performancecharacteristic.

[0019] The open-ended embodiment of FIG. 2 is similar to that of FIG. 1,except that one end of the field blank is open.

[0020] Stationary compensating coils 10-A, 10-B and 10-C can beconnected in series with the moving coil 24 in such a way that theirmagnetic fluxes work against magnetic flux created by current in themoving coil 24, thereby reducing armature reaction and resultantinductance, and leading to a faster response of the actuator. In thiscase, a new Force vs. Stroke curve would generally reflect the increasedoutput force throughout the stroke.

[0021] However, if there is a need for a “tailored” force throughoutmost of the stroke, then the compensating coils should be connected to aseparate power supply 26, as shown in FIG. 7. A position feedback device28 will be used for effective control of the compensating current valueas a function of stroke. Examples of such compensating current areprovided in the curves depicting magneto-motive force in Ampere-turnsvs. stroke for a given constant value of the output force of FIG. 3 andFIG. 4.

[0022]FIG. 3 shows the compensating coil MMF in ampere-turns, as afunction of the stroke at a constant force of 688 LB/M, for compensatingcoils 10-B and 10-C. Thus, for example, from FIG. 3 it can be seen thatat a stroke position of −0.92, the MMF for compensating coil 10-C can becontrolled to be 1000 ampere-turns so that the actuator provides a forceof 688 LB/M at that point of the stroke. The MMF to be provided in thecompensating coils 10-C and 10-B, can be identified from FIG. 3 in asimilar manner for other moving coil positions.

[0023]FIG. 4 shows the core compensating coil MMF in ampere-turns, as afunction of the stroke at a constant force of 684 LB/M, for corecompensating coil 10-A. For example, from FIG. 4 it can be seen that ata stroke position of 0.37, the MMF for core compensating coil 10-A canbe controlled to be 1500 ampere-turns so that the actuator provides aforce of 684 LB/M at that point of the stroke. The MMF to be provided inthe core compensating coil 10-A, can be identified from FIG. 4 in asimilar manner for other moving coil positions.

[0024] Comparison of the above curves of FIGS. 3 and 4 shows thatcompensating coils 10-B and 10-C, located in the spaces 14 and 16between the magnets 18 and inside walls 20 of the magnetic structure,tend to be more effective at the beginning of the stroke, whereas corecompensating coils 10-A seem to widen a constant force region toward theend of the stroke considering left-to-right movement of the coil.

[0025] The Force vs. Stroke characteristics with the two different typesof compensating coils are presented in FIG. 5 and FIG. 6. FIG. 5illustrates the force vs. stroke characteristics using compensatingcoils such as 10-B and 10-C, while FIG. 6 illustrates the force vs.stroke characteristics using core compensating coils, such as 10-A. InFIGS. 5 and 6, the curve plotting the points indicated by the diamondsymbols represents the condition where the applied MMF (I_(cc)N_(cc))equals 0 ampere-turns (A-T). The other curves represent provided MMFs asfollows: SYMBOL MMF (A-T) square box 1200 “x” 3600 “x” over-strikingvertical line 4800

[0026] From FIG. 5, it can be seen that as increasing MMF is provided inthe compensating coils 10-B or 10-C, the force towards the end of thestroke can be increased by the compensating coils in a proportionatelygreater degree than the rest of the stroke. Conversely, from FIG. 6, itcan be seen that the effect of core compensating coil 10-A on the forceproduced by the actuator is shifted toward the center of the stroke ascompared to that for compensating coils 10-C or 10-B.

[0027] Depending upon a particular application, both types of thecompensating coils 10-A and 10-B, 10-C can conceivably be utilized.

[0028] It should be noted that the utilization of such compensatingcoils increases the input power to the actuator. However, the increaseddissipation occurs in the stationary coils attached to a relativelylarge heat sink, as opposed to a moving coil which does not have such aheat sink. Further, heat generated within a moving coil doesn't change.

[0029] The compensating coil arrangement has been described as appliedto linear voice coil actuators as an example only. The same approach canbe used on rotary voice coil actuators.

[0030] The terms and expressions which have been employed herein areintended as terms of description and not of limitation, and there is nointent in the use of such terms and expressions of excluding equivalentsof the features shown and described, or portions thereof, it beingrecognized that various modifications are possible within the scope ofthe invention claimed.

What is claimed is:
 1. A voice coil actuator comprising a core having anaxis; a permanent magnet having a longitudinal axis, and positioned sothat the longitudinal axis of the permanent magnet is substantiallyparallel to the axis of the core; a moving coil positioned to interactwith the permanent magnet along the axis of the core; and a compensatingcoil positioned to interact with the moving coil, wherein amagneto-motive force in the compensating coil is controlled as afunction of a position of the moving coil.
 2. An actuator comprising acore; a permanent magnet having a longitudinal axis, and positioned sothat the longitudinal axis of the permanent magnet is substantiallyparallel to an axis of the core; a moving coil positioned to interactwith the permanent magnet along the axis of the core; and a compensatingcoil positioned to interact with the moving coil and having amagneto-motive force which is controlled as a function of a position ofthe moving coil.
 3. The actuator of claims 1 or 2, wherein thecompensating coil is positioned about the core.
 4. The actuator of claim3 , wherein the compensating coil extends along substantially the entirelength of the core.
 5. The actuator of claims 1 or 2 wherein thepermanent magnet is positioned with respect to the core to define acavity between the core and an end of the permanent magnet; and furtherwherein the compensating coil is positioned in the cavity.
 6. Theactuator of claim 5 , wherein the compensating coil fills the cavity. 7.The actuator of claim 5 , wherein a second cavity is defined at anotherend of the permanent magnet between the permanent magnet and the core,and further wherein a further compensating coil is positioned in thesecond cavity.
 8. The actuator of claim 4 , wherein the permanent magnetis positioned with respect to the core to define a cavity between thecore and an end of the permanent magnet; and further wherein a secondcompensating coil is positioned in the cavity.
 9. The actuator of claim8 , wherein a second cavity is defined at another end of the permanentmagnet between the permanent magnet and the core, and further wherein afurther compensating coil is positioned in the second cavity.
 10. Theactuator of claim 7 , further including a core compensating coilpositioned about the core.
 11. The actuator of claims 1 or 2, furtherincluding a position sensor responsive to the position of the movingcoil; and a power supply responsive to the position sensor and coupledto the compensating coil.
 12. An actuator comprising: a field blankwhich forms a housing; a core; a permanent magnet having a longitudinalaxis, positioned in the housing and so that the longitudinal axis of thepermanent magnet is substantially parallel to an axis of the core; amoving coil positioned to interact with the permanent magnet along theaxis of the core; and a plurality of compensating coils positioned tointeract with the moving coil and controlled as a function of a positionof the moving coil, including a core compensating coil positioned aboutthe core; a first compensating coil positioned in a cavity formedbetween an end of the permanent magnet and the housing.
 13. The actuatorof claim 12 , further including a second compensating coil positioned ina second cavity formed between an other end of the permanent magnet andthe housing.
 14. The actuator of claim 13 , further including a positionsensor responsive to the position of the moving coil; and a plurality ofpower supplies, each responsive to the position sensor, and coupled toprovide power to a corresponding one of the core compensating coil, thefirst compensating coil, and the second compensating coil, as a functionof the stroke of the moving coil.
 15. The actuator of claim 4 , whereinthe actuator is closed-ended.
 16. The actuator of claim 4 , wherein theactuator is open-ended.
 17. The actuator of claim 10 , wherein theactuator is closed-ended.
 18. The actuator of claim 10 , wherein theactuator is open-ended.
 19. The actuator of claim 14 , wherein theactuator is closed-ended.
 20. The actuator of claim 14 , wherein theactuator is open-ended.
 21. The actuator of claims 1 or 2, wherein theactuator has a rectangular cross section.
 22. The actuator of claims 1or 2, wherein the actuator has a cylindrical shape.
 23. A voice coilactuator comprising: a field blank which forms a housing; a core; apermanent magnet having a longitudinal axis, and positioned so that thelongitudinal axis of the permanent magnet is substantially parallel toan axis of the core; a coil positioned about the core to be moveablealong and to interact with the permanent magnet along the axis of thecore, wherein magnetic flux is produced in an air gap between themoveable coil and the permanent magnet; a compensating coil positionedto interact with the moveable coil; and a power supply coupled to thecompensating coil so that magneto-motive force is produced in thecompensating coil of a magnitude which is a function of positions of themoveable coil along the core to alter the magnetic flux in the air gap.24. The voice coil actuator of claim 23 , wherein the compensating coilis positioned about the core.
 25. The voice coil actuator of claim 23 ,in which the core, the permanent magnet, the moving coil and thecompensating coil are positioned in the housing formed by the fieldblank, and wherein the compensating coil is further positioned at an endof the permanent magnet along the longitudinal axis of the permanentmagnet and in a cavity between the permanent magnet and the field blank.26. The voice coil actuator of claim 25 , wherein the voice coilactuator has a rectangular cross section.